Variable gain feedback amplifier



July 16, 1940. J. M. WEST VARIABLE GAIN FEEDBACK AMPLIFIER Filed March l0, 1938 LOG. FREQ. 7n-C- W5/vm@ By J. M. WEST A TTORNEV Patented July 16, 1940 PATENT OFFICE VARIABLE GAIN FEEDBACK 'AMPLIFIER Julian M. West, Ridgewood, N. J., assigner Ato Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of `New` York, i

Application March 10, 1938,?Srial No. 195,063

sclaims. (c1.nel-1.71)jV f" The present invention relates to electrical Wave amplifiers and particularly to broad band amplifiers of adjustable gain.

A general object of the invention is to provide gain adjustment in a stabilized feedback amplid fier Within the operating frequency band in effective and satisfactory manner.

It has been proposed heretofore to vary the gain ofv a stabilized feedback amplifier by use 1Q of a condenser type potentiometer in the feedback or path. This has proved effective as a means for producing gain changes that are flat throughout the transmitted band, which may be,

by Way of example, the band used for multiplex 1g carrier transmission on lines or in radio or other high frequency systems. It is found, however', that the use of a condenser potentiometer in shunt across the feedback loop as heretofore proposed raises a difficulty in applying the principles of feedback amplifier design disclosed in H. W. Bode Patent No. 2,123,178, issued July 12, 1938, in that it has a tendency to reduce or destroy the phase margin at the upper limiting frequencies of the'wave components traversing the a loop, thereby limiting the amount of feedback that can be used. K

In accordance with the present invention the convenience and effectiveness of the condenser type potentiometer as a flat gain control are rep tained but the high frequency phase margin is not reduced harmfully, if at all.

In a specific embodiment the invention comprises a series impedance in the condenser shunt arm of such character as not to impair the operation of the Vcondenser potentiometer over the useful band while permitting the total shunt impedance to hold up to the required value in the region of the upper limiting frequencies traversing the loop. 40 The invention and its various features will be more clearly understood from the following detailed description taken in connection with the drawing, in which:

Fig. l is a schematic circuit diagram of an amplifler equipped with one form. of gain controlling circuit in accordance with the invention;

Fig. 2 shows a modification according to the invention; and

Fig. 3 shows curves illustrating an example of operating characteristic of an amplifier according to the invention.

Fig. l shows a three-stage amplifier comprising triodes I, 2 and 3 with interstage networks N1 and N2. It-Will be understood that as regards the amplifier, the showing is general and that tubes other than triodes are contemplated as Well. Also the direct current supplies and connections are omitted for simplicity since any known .or usual circuits for these purposes are to be assumed. An input I0 leads up to the grid of stage land anoutput or load circuit II is lcoupled lto thelast anode circuit.

The amplifier is provided with a gain reducing feedback path `indicated as leading from a suitable point in -`the output side of stage 3 by Way of leadltZ, blocking condenser I3 and resistance I4 to the cathode of stage I, the grid ofthis stage being connected toa point in resistor I4 remote from the cathode. This feedback connectio'nisI of the type and for the purposes generally disclosed' in U. Si. Patent 2,102,671 of H. S. Black issued December 21, 1937. That is, the feedback voltage is in a direction andyof an amount to reduce substantially the gain of the amplifier in order (among other things) toincrease its linearity and stability of amplification. el".

lAs an example of one use to which the amplilier of Fig. 1 may be put, it may be inserted in a broadband transmission line such as ay multipleX` carrier telephone line to amplify in suitable manner the waves'comprised in the entire fre- :fi

quency range traversing theline. This` range is referred to as the useful or utilized range or band. Within this band the resultant or overall amplification is equal to the amplification Without feedback minus the reduction in amplification due to the feedback. In the usual terminology, the gain or amplification with zero feedback is a and the gain `with negative feedback is equal to kl/f7 It is convenient to refer to the amplifying portion or forwardly transmitting portion of the circuit as the ,1t-circuit or /i-path 'and the feedback circuit as the-circuit or ,fi-path.v (Exact definitions are given in vBlacks patent, cited above, to which reference may be made.)

When Iftl is very `large compared to unity, the overall gain is substantially proportional to Y so that the overall gain, vshape of characteristic, etc. are determined by the design or adjustment of the -circuit. For the purpose of adjusting ,M

the gain of the amplifier of Fig. 1 (still keeping |a| large compared to unity) the value of is controlled bythe condenser potentiometer comprising condensers I3'and I5. The impedance of I3 Within the band is very high compared with that densers I3 and I5 (neglecting for the moment l resistance I'I) form a potential divider, by which a proportionate amount of the voltage in the feedback` circuit is applied to the grid of tube I under control of variable condenser I5'. In

' parasitic capacities.

this way the overall gain of the amplifier is varied and moreover the gain changes are independent of frequency Within the utilized band since I and I3 are both capacitive impedances Which behave similarly over the frequency band.V

It will be understood that the condensersl3r anti-resonate with the parasitic capacities and increase the feedback (which turns positive)` in the upper limiting frequency range.

This is illustrated in the curves of Fig. 3 Where characteristic A gives the gain over the useful band (illustrated as extending from about 0.06 megacycle to 2 megacycles) and up to 30 megacycles, the topmost frequency of interest, corresponding to one gain setting of the condensers I3 vand I5. Curve B is a similar curve acorresponding to a different gain setting. These curves are seen to fall off rapidly just above the useful band and then to assume a definite slope and as the upper limitingfrequencies are approached, the slope changes rather abruptly upward. This rise is due to the anti-resonance of the shunt inductance of network I8 and the However, if it were not for the presence of resistance I'I,fthe capacity I5, which has substantially zero impedance at 30 megacycles, would shunt out the anti-resonant impedance of the feedback path at 30 megacycles and resultin a much less favorable shape of curve. For example, the curves would be falling instead of rising or holding up as the A30 megacycle point is approached. As disclosed in the Bode patent referred to, such shape of /i gain curve (Without resistance I'I) would be accompanied by a phase shift characteristic that would reach zero at some frequency, Well below 30 megacycles, at which the a gain might still be positive. This wouldr result in a less favorable singing margin.

By use of resistance I'I, however, curves generally similar to curves A and B are obtainable. The corresponding phase shift characteristic A' br B does not turn down and reach zero until well past the frequency at'which the gain` has become negative. This is due to the shunt loss introduced across the feedback path by network I8 assisted by parallel resistance II. In one case resistance I1 had a value of 500 ohms which within the useful band was small compared with the impedance of condensers I3 and I5 each of which may be of the order of something less than 100 micromicrofarads throughout the entire range of adjustment.

lIt is clear from what has been said that the purpose of resistance I'I is rto make the shunt impedance across the feedback circuit high at the highest frequencies of interest while preferably .having negligible effect upon the gain adjustment inthe useful band. Various types of networks v y'will occur to those skilled in the art which could be used to good advantage in place of the simple.

resistance I'I. A simple type of network is given for illustration in Fig. 2 in which inductance I9 has the effect of shunting out the resistance I I in the useful band where I9 has very low impedance, but of eifectively introducing resistance I1 at very high frequencies where I9 has high impedance. More complicated networks to give .closer approximation to the ideal or desired shape of curve may be used.

The invention is not to be construed as limited by the numerical values given nor by the circuit details since `these are for illustration. The

4 scope of the invention is defined in the claims.

What is claimed is: f

l. In a negative feedback broad band amplifier having a forwardly transmitting portion and a feedback circuit, said amplifier possessing a phase margin against singing at high frequencies outside the utilized band, means comprising a variable capacity in shunt relation to said feedback circuit for varying the overall gain within the utilized band and an impedance in series with said capacity for preventing the capacity from reducing the phase margin at the said high frequencies, said impedance having negligibly small effect on the gain Within the utilized band.

2. In a broad band feedback amplifier having a ,c portion and a portion, said amplier possessing a phase margin against singing at the high frequency end of .its transmission range, means for controlling the amplifier gain Within the utilized Aband comprising an adjustable capacity in shunt relation to said portion, and a resistive impedance in series with said adjustable capacity, the value of the resistive impedance be ing too small to have a controlling effect on the gain within the utilized frequency band but high enough at frequencies approaching the high frequency limit of the c'gain characteristic to prevent loss of the aforesaid phase margin.

3. In a broad band negative feed back ampli fier having an input circuit and an output circuit, a feedback path from the output to the input feeding backwaves in gain reducing sense, a series capacity in said path and a shunt capacity across said path, said two capacities forming agpotential divider for controlling the magnitude o high compared to that of said shunt capacity and of parasitic shunt capacities in the region of the upper limiting frequencies traversing the circuit.

4. An amplifier according to claim 3, in which said impedance comprises a resistance shunted by a reactive impedance of low value of impedance compared to the resistance in the operating range but of much higher value than the resistance in the region of the upper limiting frequencies traversing vthe circuit.

5. In a broad band ampliiier circuit, a negative feedback circuit therefor, said amplifier possessing a phase margin against singing in the region 'of the upper limiting frequencies traversing the circuit, means for controlling the gain of the amplifier comprising a condenser in shunt `relation to the feedback circuit, and an impedance in series with said condenser the value of impedance of which is smally compared to that of said condenser within the utilized frequency band but t'he effective total resistance of which is high enough at frequencies in the region of the upper limiting frequencies traversing the circuit to Drevent loss of the aforesaid phase margin.

. JULIAN M. WEST. 

